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LT3845IFE Datasheet(PDF) 10 Page - Linear Technology |
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LT3845IFE Datasheet(HTML) 10 Page - Linear Technology |
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10 / 26 page  LT3845 10 3845fd APPLICATIONS INFORMATION Overview The LT3845 is a high input voltage range step-down synchronous DC/DC converter controller IC that uses a programmable constant frequency, current mode archi- tecture with external N-channel MOSFET switches. The LT3845 has provisions for high efficiency, low load operation for battery-powered applications. Burst Mode operation reduces total average input quiescent currents to 120μA during no load conditions. A low current shutdown mode can also be activated, reducing quiescent current to 10μA. Burst Mode operation can be disabled if desired. A reverse-current inhibit feature allows increased efficien- cies during light loads through nonsynchronous operation. This feature disables the synchronous switch if inductor current approaches zero. If full time synchronous opera- tion is desired, this feature can be disabled. Much of the IC’s internal circuitry is biased from an internal linear regulator. The output of this regulator is the VCC pin, allowing bypassing of the internal regulator. The associated internal circuitry can be powered from the output of the converter, increasing overall converter efficiency. Using externally derived power also eliminates the IC’s power dissipation associated with the internal VIN to VCC regulator. Theory of Operation (See Block Diagram) The LT3845 senses converter output voltage via the VFB pin. The difference between the voltage on this pin and an internal 1.231V reference is amplified to generate an error voltage on the VC pin which is used as a threshold for the current sense comparator. During normal operation, the LT3845 internal oscillator runs at the programmed frequency. At the beginning of each oscillator cycle, the switch drive is enabled. The switch drive stays enabled until the sensed switch current exceeds the VC derived threshold for the current sense comparator and, in turn, disables the switch driver. If the current comparator threshold is not obtained for the entire oscillator cycle, the switch driver is disabled at the end of the cycle for 350ns, typical. This minimum off-time mode of operation assures regeneration of the BOOST bootstrapped supply. Power Requirements The LT3845 is biased using an internal linear regulator to generate operational voltages from the VIN pin. Virtually all of the circuitry in the LT3845 is biased via this internal linear regulator output (VCC). This pin is decoupled with a low ESR, 1μF capacitor to PGND. The VCC regulator generates an 8V output provided there is ample voltage on the VIN pin. The VCC regulator has approximately 1V of dropout, and will follow the VIN pin with voltages below the dropout threshold. The LT3845 has a start-up requirement of VIN > 7.5V. This assures that the onboard regulator has ample headroom to bring the VCC pin above its UVLO threshold. The VCC regulator can only source current, so forcing the VCC pin above its 8V regulated voltage allows use of externally derived power for the IC, minimizing power dissipation in the IC. Using the onboard regulator for start-up, then deriving power for VCCfromtheconverteroutputmaximizes conversion efficiencies and is common practice. If VCC is maintained above 6.5V using an external source, the LT3845 can continue to operate with VIN as low as 4V. The LT3845 operates with 3mA quiescent current from the VCC supply. This current is a fraction of the actual VCC quiescent currents during normal operation. Additional current is produced from the MOSFET switching currents for both the boosted and synchronous switches and are typically derived from the VCC supply. Because the LT3845 uses a linear regulator to generate VCC, power dissipation can become a concern with high VIN voltages. Gate drive currents are typically in the range of 5mA to 15mA per MOSFET, so gate drive currents can create substantial power dissipation. It is advisable to derive VCC and VBOOST power from an external source whenever possible. The onboard VCC regulator will provide gate drive power for start-up under all conditions with total MOSFET gate charge loads up to 180nC. The regulator can operate the LT3845 continuously, provided the power dissipation of the regulator does not exceed 250mW. The power dissipation of the regulator is calculated as follows: PD(REG) = (VIN – 8V) • (fSW • QG(TOTAL) + 3mA) |
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